1. Field of the Invention
The present invention relates to an image processing device, and to an image display device provided with such an image processing device.
2. Description of the Prior Art
In recent years, as electronic devices designed mainly to process color images become popular, it has become easy to handle color images not only in specialized fields such as computer graphics-based designing but also in general office work. However, when the data of a color image created on a personal computer or with a digital still camera is transferred by e-mail so that the receiver stores the received data on a HDD device, a floppy disk, or a recording medium built in a digital still camera and then outputs it as a color image, the colors usually do not match between the sender and the receiver. This makes it difficult to check the colors of an image on a monitor. As a means to solve this inconvenience, color management systems have been devised and have been attracting much attention.
A color management system aims to eliminate color differences from one device to another by the use of a common color space. This is based on the thought that colors identified with identical coordinates in an identical color space appear identical (i.e. those colors match), and accordingly a color management system evaluates all colors in an identical color space and attempts to match colors by making their coordinates identical. One method commonly used today is to use a CIE-XYZ color space as a color space and correct color differences from one device to another by the use of XYZ tristimulus values, i.e. coordinates identifying specific points within the color space. A technique for achieving color matching based on this method is disclosed, for example, in Japanese Patent Application Laid-Open No. H11-134478.
However, inconveniently, even though a color management system as described above achieves color matching under specific ambient-light conditions, a variation in the environmental and other conditions under which an image is observed causes a change in how the image appears.
FIG. 10 is a diagram illustrating a case in which identical images displayed on different personal computers in different environments are observed by the use of a color management system. Here, user A (sender) transmits an image 102 displayed on the monitor 101 of the sender-side personal computer to user B (receiver). The image transmitted from user A is received by user B, and is displayed as an image 202 on the monitor 201 of the receiver-side personal computer.
In such a case, there is almost no probability that the ambient-light conditions 103 around the monitor 101 of the sender-side personal computer are identical with the ambient-light conditions 203 around the monitor 201 of the receiver-side personal computer. Thus, in this case, even though the color management system achieves color matching between the images 102 and 202 under specific ambient-light conditions, a variation in ambient-light conditions causes a change in how the images appear, destroying color matching.
Moreover, in cases where transmissive liquid crystal display devices are used as the monitors 101 and 201 of the personal computers mentioned above, the environmental and other conditions under which the images are observed may vary because of variations with time in the characteristics of the color filters of the transmissive liquid crystal display devices, or variations with ambient temperature or with time in the characteristics of the backlight sources thereof. Such variations also cause a change in how the images appear, and thus destroy color matching. The factors that cause variations in the environmental and other conditions under which the images are observed include variations with time in the brightness and chromaticity of the backlight, variations with temperature in the brightness of the backlight, and the like.
FIG. 11 is a diagram showing the variation with time of the brightness (i.e. the brightness preservation ratio) of the backlight of a typical transmissive liquid crystal display device. In this figure, along the horizontal axis is taken the accumulated lit (“on”) period of the backlight source, and along the vertical axis is taken the brightness preservation ratio thereof. The brightness preservation ratio is the ratio of the current brightness of the backlight source at a given time to the initial brightness (100%) thereof. As shown in this figure, the brightness preservation ratio decreases with the accumulated lit period. Generally, the period over which the brightness preservation ratio of the backlight source reduces to 50% is evaluated as the operating life thereof.
FIG. 12 is a diagram showing the variation with time of the chromaticity (i.e. the chromaticity shift) of the backlight of a typical transmissive liquid crystal display device. In this figure, along the horizontal axis is taken the accumulated lit period of the backlight source, and along the vertical axis is taken the chromaticity shift (X, Y) thereof. The chromaticity shift (X, Y) is an important parameter that indicates the degree in which the current chromaticity of the backlight source at a given time has varied from the initial chromaticity thereof. Generally, the chromaticity, represented by values X and Y, of the backlight source increase with the accumulated lit period thereof.
FIG. 13 is a diagram showing the temperature dependence of the brightness of the backlight of a transmissive liquid crystal display device. In this figure, along the horizontal axis is taken the tube wall temperature of the backlight source, and along the vertical axis is taken the brightness thereof. As shown in this figure, the brightness of the backlight source varies greatly with the tube wall temperature thereof. The tube wall temperature of the back light source varies with the period over which it has been lit and with ambient temperature.
FIG. 14 is a diagram showing an example of the chromaticity coordinate system of a color filter of a transmissive liquid crystal display device. In this figure, along the horizontal axis is taken the chromaticity x of the color filter, and along the vertical axis is taken the chromaticity y thereof. In this figure, points A, B, C, and D indicate the green point, red point, blue point, and white point, respectively, and the triangle enclosing points A, B, C, and D represents the chromaticity (x, y) of the color filter.
The parameters mentioned above (the brightness and chromaticity of the backlight, the chromaticity of the color filter, and the like) vary differently from one transmissive liquid crystal display device to another. Therefore, even if color matching is achieved between images under specific conditions, it is liable to be destroyed by a variation in the environmental and other conditions under which the images are observed, or a variation with time in those parameters.
Moreover, on different personal computers, identical images are displayed and observed by their users under different environmental and other conditions.
Therefore, even if a color management system achieves color matching between images displayed on different personal computers under specific anbient-light conditions and at a given time, it is difficult to maintain the color matching between the images against the deterioration with time of the devices used, because different personal computers differ in the period over which their monitor has been used and in their characteristics.